I don’t see what the problem is. Finn, like that stormtrooper he faces in TFA, has hand to hand combat training, that staff that the stormtrooper uses is like a lightsaber, I don’t see anything wrong with him being able to use it. Its not like hes deflecting blaster bolts with it.
He got his ass handed to him by a stormtrooper with a crowd-control staff. If Lando was in a gunfight, you wouldn’t take away his blaster and give him a lightsaber. For some reason Maz had faith Finn could use the Jedi weapon, or she’s got a weird sense of humor.